"With shame and repentance, we acknowledge as an ecclesial community that we were not where we should have been, that we did not act in a timely manner, realizing the magnitude and the gravity of the damage done to so many lives," he writes.

The Pope's message quotes a passage from the Bible in Corinthians that says "if one member suffers, all suffer together" and calls for the Church to come to grips with the reality of what has happened in a "comprehensive and communal way".

"If, in the past, the response was one of omission, today we want solidarity, in the deepest and most challenging sense, to become our way of forging present and future history.

"It is essential that we, as a Church, be able to acknowledge and condemn, with sorrow and shame, the atrocities perpetrated by consecrated persons, clerics, and all those entrusted with the mission of watching over and caring for those most vulnerable. Let us beg forgiveness for our own sins and the sins of others."

What are the latest cases?

The letter's release comes days before the Pope is due to travel to Ireland to address the Church's World Meeting of Families.

US church officials described as credible allegations that he sexually assaulted a teenager nearly 50 years ago. He says he has no recollection of this.

In May, Archbishop Philip Wilson became the most senior Catholic in the world to be charged with concealing child sexual abuse in Australia in the 1970s.

What has the reaction been?

Margaret McGuckin from Survivors & Victims of Institutional Abuse, a historic abuse charity based in Northern Ireland, said the Pope's response was "too little too late" and said "nothing will change" with the letter.

Anne Barrett-Doyle, co-director of clerical abuse tracking website BishopAccountability.org, said there needed to be "more actions, less words" from Pope Francis.

"He needs an effective discipline process for bishops and religious superiors who are known to have enabled abuse," she said.

Irish campaigner Marie Collins, who was abused by a priest as a teenager, welcomed the condemnation of Church cover-ups, but said more concrete action on accountability was needed.

Statements from Vatican or Pope should stop telling us how terrible abuse is and how all must be held accountable. Tell us instead what you are doing to hold them accountable. That is what we want to hear. “Working on it” is not an acceptable explanation for decades of “delay”

The condemnation of clericalism in the letter is good to see as it plays a big part in the ignoring of the laity, survivors and experts. It gives rise to the ease with which church leaders can feel comfortable protecting fellow clerics despite their crimes against children

Voice of the Faithful, a group that was formed after revelations of abuse in Boston in 2002, welcomed Monday's letter but reiterated the necessity of full accountability for offenders and those who covered abuse up.

"Although we applaud the Pope's expressions of regret and sorrow, as always, we find ourselves anxiously anticipating action to back up his words," the statement said.